The present invention relates to pavement markers of the cube corner reflex reflector type which are cleaned by the action of vehicular traffic on the roadway contacting the reflector, and in particular to such pavement markers which are suitable for use in snow areas and are, therefore, constructed so as to protect the reflector from contact with snowplow blades.
Pavement markers have become more widely accepted as permanent installations for providing visible signals which mark traffic lanes and control the flow of traffic on roadways in connection with, or in place of, conventional painted traffic lines. While a large number of such markers employ reflectors which reflect light emanating from oncoming vehicles to provide a visible signal to the operators of such oncoming vehicles, other markers have been proposed which utilize an independent light source, such as an electric lamp located within the marker, to provide a signal visible from oncoming vehicles. The term "signal means" is employed herein to denote any such marker employing a reflector, a lamp or another light source or any arrangement which provides the desired visible signal.
A snowplowable version of such a prior art pavement marker is disclosed in U.S. Pat. No. 3,790,293, issued to S. A. Heenan et al. on Feb. 5, 1974, and U.S. Pat. No. 3,809,487, issued to R. M. Flanagan on May 7, 1974, both of which patents are assigned to the assignee of the present invention. In the arrangements used in those patents, a base member of relatively high-strength material, such as metal, includes a pair of laterally spaced-apart keels which are permanently affixed to the roadway surface by insertion in grooves cut in the pavement, and a reflector body of synthetic resin material is affixed to the base member for selective removal and replacement without destruction of the base member. The base member is provided with inclined ramps for protecting the reflector body from encounters with snowplow blades.
In these prior art snowplowable pavement markers, an attempt was made to minimize the height of the pavement marker above the roadway surface by minimizing the height of the reflector body carried by the base, thereby to minimize the impact forces imparted to snowplow blades as they passed over the pavement marker. Indeed, in these prior art snowplowable pavement markers the maximum height of the marker above the roadway surface had been reduced as far as possible with existing reflex reflector bodies and installation techniques, consistent with obtaining satisfactory visibility of the pavement marker, but could not be reduced below about 0.72 inches above the pavement.
It has also been recognized in connection with these prior art pavement markers that the angle between the roadway surface and the inclined ramps of the base member should be minimized to minimize the impact forces imparted to the pavement marker and to the surrounding pavement and to snowplows by impact of snowplow blades with the inclined ramps of the pavement marker. While theoretically the ramp angle could be reduced as low as desired, the lower the angle, the longer the ramp would have to be to maintain the same maximum height and, accordingly, the longer the keel members and the longer the grooves or recesses that would have to be cut in the pavement. The longer the grooves, the greater the weakening of the pavement and the greater the time and expense required to form the grooves. Furthermore, the longer the base member, the heavier and more expensive it is. Thus, these factors serve to practically limit the ramp angle that could be obtained with these prior art pavement markers to no lower than six degrees.
In addition, the prior art pavement markers were monodirectional devices. While bidirectional reflector bodies were available, in order to mount them in a metal base member for protection from impact with snowplow blades, it would be necessary to have inclined ramps extending from the reflector body in both directions. Thus, if the same ramp angle and maximum height above the roadway surface were to be maintained, it would be necessary to virtually double the length of the base member, with the attendant disadvantages discussed above.
It has been suggested in the prior art to minimize the height of the pavement marker by partially recessing the reflector elements below the level of the roadway surface as, for example, in U.S. Pat. No. 2,260,498, issued to L. M. Wise on Oct. 28, 1941. The Wise pavement marker is a generally cylindrical body adapted to be embedded in the pavement, with a part-conical upper surface extending above the level of the roadway surface and providing an inclined surface to deflect snowplow blades from contact with the reflector bodies. But the inclined surface of the Wise pavement marker apparently is disposed at an angle in excess of 25.degree. with the roadway surface, an angle which has proven in practice to be far too great for satisfactory operation, since the impact forces of snowplow blades against the pavement marker are so great that they destroy the pavement marker and/or severely damage the surrounding pavement. While theoretically the ramp angle of the Wise pavement marker could be reduced by increasing the diameter of his pavement marker, the enlarged recesses which would be required would unacceptably impair the integrity of the roadway and would be unduly expensive, and the size and weight of the pavement marker itself would be increased to unacceptable levels. Furthermore, the cross sectional outline of the Wise pavement marker body is not conducive to ready insertion in a recess which is cut or drilled in a finished pavement.